1. Field of the Invention
This invention relates generally to solid state imager devices and specifically to a solid state imager device having an electronic shutter function suitable for use in a video camera, an electronic still camera, etc.
2. Description of the Prior Art
It has been desired in the art that a solid state imager device be able to control its exposure time by electronic means provided therein so that when the solid state imager device is used in, for example, a video camera, the use of a mechanical shutter, which is common in the prior art, becomes unnecessary. Without a mechanical shutter, a video camera could be made small in size, light in weight and low in cost.
There has already been proposed in the art a solid state imager device which permits selection between two exposure times, one long and one short. This solid state imager device has one of the two exposure times chosen to be 1/30 second, which is equal to one frame period, and the other exposure time chosen to be about 1/2000 second, which is equal to a vertical blanking period.
In accordance with this solid state imager device, when the 1/30 second exposure time is selected, signal charges accumulated in the light receiving section during one frame period are read out as an image signal. On the other hand, when the 1/2000 second exposure time is selected, signal charges accumulated before a vertical blanking period are read out to the vertical register section and discharged by a high speed transfer while signal charges newly accumulated only for the vertical blanking period are read out as an image signal.
However, the above-discussed solid state imager device is limited in that it only allows selection between two exposure times so that, even when a video camera is equipped with such solid state imager device, the video camera still requires a mechanical shutter. Moreover, since a high speed transfer is required to discharge the signal charges accumulated before a vertical blanking period to the vertical register, the solid state imager device consumes excessive electric power.